In recent years, LEDs (light-emitting diodes) have been used widely due to good light-emitting quality and high light-emitting efficiency. LEDs have been developed capable of emitting red, green, and blue light, and LEDs have been increasingly used for various applications such as a full-color LED display, so that color performance of the full-color LED display can be increased. Typically, the red, green, and blue lights respectively generated by the red, green, and blue LEDs mixed to form colored light with high brightness and contrast, so as to display some information for viewer.